This invention relates to an improved distillation system for recovering motor fuel grade anhydrous alcohol from fermentation or synthetic feedstocks.
The use of anhydrous alcohol (99.5 vol.% ethanol) has become an important consideration as a means of saving gasoline produced from high-cost crude oil. It is a well-established fact that up to 20 percent anhydrous ethanol can be blended with gasoline to obtain a relatively high octane antiknock fuel which can be used for internal combustion engines. With some engine modification, anhydrous ethanol can be used as the fuel directly.
Growing requirements for anhydrous ethanol for use in motor fuel gasoline blends require systems that operate with a minimum of energy and that are also reliable in continuous operation. Although production and blending of ethanol with gasoline have been practiced in different countries during the past forty years, the use of ethanol in such blends has been limited because of the relatively high costs of production.
The conventional distillation system for recovering motor fuel grade anhydrous ethanol from a dilute feedstock, such as fermented beer or synthetic crude alcohol, utilizes three towers. In the first tower the feedstock containing, for example, 6 to 10 vol.% ethanol is subjected to a preliminary stripping and rectifying operation in which the concentration of water is materially reduced and concentrated ethanol stream is removed which contains on the order of 95 vol.% ethanol, thereby approaching the ethanol-water azeotrope composition of about 97 vol.% ethanol. The concentrated ethanol stream is next subjected to azeotropic distillation in the second or dehydrating tower using a suitable azeotropic or entraining agent, usually benzene or a benzene-heptane mixture. This results in removal of most of the remaining water, and the desired motor fuel grade anhydrous ethanol product (99.5 vol.%) is recovered from the dehydrating tower. The third tower of the system comprises a stripping tower in which the benzene or other azeotropic agent is recovered from the water-rich phase following condensation and decantation of the azeotropic overhead stream from the dehydrating tower.
One of the key elements in the high operating cost of the above-described conventional distillation system is the high thermal energy requirements of the system, particularly steam consumption. The conventional system also has other serious shortcomings which detract from the commercial feasibility of the use of anhydrous ethanol as motor fuel. For example, the stripper-rectifier tower is occasionally operated under superatmospheric pressure which results in higher temperatures which in turn cause rapid fouling and plugging of the trays. As a consequence, periodic interruption of the operation is necessary to permit cleaning of the tower with resultant high maintenance costs. Futhermore, the conventional system does not include adequate provision to overcome the operating difficulties and product quality problems caused by the presence of higher boiling and lower boiling impurities in the feedstock.
Certain proposals have been made in the prior art to reduce the thermal energy requirements of the system. For example, in 1931-32 the Ricard et al U.S. Pat. Nos. 1,822,454 and 1,860,554 disclosed the use of higher pressures in the first tower than in the other towers and the condensation of the high pressure overhead vapors from the first tower to supply heat to the other towers. However, the energy savings which can be realized by the Ricard et al proposals fall short of the economies required under present day conditions. Moreover, the Ricard et al proposals do not meet the other objections to the conventional system discussed above.